This invention relates to plasma arc welding and relates, more particularly, to the control of plasma gas flow through the orifice of a plasma welding torch.
Plasma arc welding apparatus with which this application is concerned includes a plasma torch positionable in working relationship to a workpiece, means for moving the workpiece and torch relative to one another along a desired welding seam, a plasma gas source and an electric power supply. The torch includes a body defining a flow passage for plasma gas and an electrode supported within the flow passage. The torch flow passage is connected to the plasma gas source by means of a gas line for receiving plasma gas and defines an orifice through which the plasma gas exits the torch. The electric power supply is operatively connected between the electrode and the workpiece for passage of a current and an arc between the workpiece and the electrode, which arc is directed through the torch orifice so that plasma gas approaching the orifice exits the orifice as a high-temperature jet stream.
It is known that for some plasma welding applications, such as keyhole welding, welding factors such as the rate of gas flow through the torch orifice, the weld current and the relative speed of movement between the torch and workpiece must be accurately controlled to ensure a suitable weld in the workpiece. To this end, drive means are utilized for moving the torch and workpiece relative to one another, current control systems are connected to the power supply, and gas flow slope control means are operatively associated with the gas source. Furthermore, a computer is commonly connected to the drive means, current control systems and slope control means for automatically controlling the aforementioned factors during a welding operation.
Slope control means of conventional automatically-controlled welding apparatus commonly includes a slope control valve connected in the plasma gas flow line between the plasma gas source and the torch flow passage and remotely of the plasma torch. Because of such remote positioning of the control valve, any adjustment of the plasma gas flow necessarily occurs remotely of the torch. Such remote positioning provides a disadvantage in that an adjustment of the gas flow at the torch orifice in response to an adjustment of the valve is necessarily delayed. Furthermore, the portion of the gas line extending between the control valve and the torch is typically large in capacity when compared to the relatively small gas flows involved so that the aforesaid gas line portion provides, in effect, a gas reservoir from which plasma gas can seep into the torch unchecked. Both the time delay involved between the valve adjustment and a corresponding adjustment in the plasma gas flow at the torch and the reservoir effect of the gas flow line present difficulties in accurately controlling the flow of plasma gas through the torch.
It is an object of the present invention to provide a new and improved apparatus for accurately controlling gas slope through a torch during a plasma welding operation.
Another object of the present invention is to provide such an apparatus which circumvents difficulties associated with the control of gas slope in conventional plasma welding apparatus.
Still another object of the present invention is to provide such an apparatus which is economical to manufacture and effective in operation.
Yet still another object of the present invention is to provide a new and improved torch for plasma welding.